E: Iris - The iris is the colored portion of your eye that surrounds the pupil. It can open and close, making the pupil bigger or smaller in response to the amount of light that passes through the cornea.
D: Pupil - The round, black part of your eye is the pupil. It gets smaller (or constricts) in bright light and grows larger (or dilates) in the dark to absorb as much light as possible.
B: Lens - After exiting the pupil, light rays then pass through the lens. The lens can bend and change shape to better focus the light towards the retina. This makes your “picture” as clear as possible.
A: Vitreous Humor - The vitreous humor (or vitreous gel) comprises the majority of the eye. It’s a clear gel-like substance that light has to pass through to get to the retina. Its main function is to give the eye its shape and keep other parts in place.
H: Retina - The back of the eye is lined with a thin layer of tissue called the retina. When we see things, the picture is actually processed as upside-down! In our brains, we turn the image right-side-up again, just like it is in reality. The retina contains millions of tiny nerve cells that we call rods and cones. Cones detect color while rods can detect motion and help us see in dim lighting.
G: Optic Nerve - Once the cells in the retina have converted light into electrical impulses, the signals are sent through the optic nerve to our brains. One interesting thing about our eyes is that information from the right eye is sent to the left half of your brain, and vice versa!
Night and Color Vision
The cone cells are not as sensitive to light as rods. However, each cone is sensitive to one of these three colors: green, red or blue. Signals from the cones are sent to the brain, which then interprets and translates this information into the perception of color. Unfortunately, cones only work in bright light. That's why you can’t see color very well in the dark. So, the cones are used for color vision and are better suited for detecting fine details. The human retina contains about six million cone cells!
Since the the cone cells are not as sensitive to light, it can be very difficult to see colors in dim lighting. Try the experiment below to test your color vision in dim light!
Some people can’t tell certain colors from others, regardless of the light; these people are referred to as "colorblind." Someone who is colorblind wouldn’t have a particular type of cone in the retina, or one type of cone may be weaker than the others. There are many different types of color blindness, but the most common is red-green. People with red-green color blindness see shades of red and shades of green as the same color. This illustration from Edward Scripture’s 1895 book, “Thinking, Feeling, Doing” shows what the American flag would look like to people with different types of color blindness!
Here's what you will need:
- A flashlight
- A dark room; either one with no windows, or you will have to do the experiment at night
- Some pieces of colored paper
- A friend to help you
Here's what to do:
- Make sure the room can be made completely dark. Turn on the flashlight, and set it up in one corner of the room
- Turn your back to the flashlight in the opposite corner of the room
- Have your friend hold up a piece of colored paper. Without seeing the paper beforehand, try to guess the color of the paper in very dim lighting.
- If you cannot guess the color, move one step closer to the flashlight and try again.
- Keep taking single steps toward the flashlight until you get the color right.
- Repeat this process with the other pieces of colored paper. Were any of the color is easier to see in dim light?
Color blindness can also be detected by looking at designs with different colored dots on them, such as the one shown below. Can you see a number?
“How Your Eyes Work”. American Optometric Association. https://www.aoa.org/patients-and-public/resources-for-teachers/how-your-eyes-work
“Ishihara Color Test”. Color Blindness. color-blindness.com. Accessed on 10/12/17.
Kratochvil, Petr. “Closeup Eye”. Released into the public domain. Uploaded on 10/12/17 from publicdomainpictures.net
National Eye Institute, Department of Health and Human Services. “Eye Diagram without Text”. Released into the public domain. Uploaded on 10/12/17 from commons.wikimedia.org
“Night Vision Scene”. Released into the public domain. Uploaded on 10/12/17 from publicdomainpictures.net
Montag, Ethan. “Eye Receptors: Rods and Cones”. Released into the public domain. Uploaded on 10/10/17 from commons.wikimedia.org
Scripture, Edward Wheeler. “Thinking, Feeling, Doing”. Released into the public domain. Uploaded on 10/12/17 from commons.wikimedia.org
“Ishihara Plate 9”. Released into the public domain. Uploaded on 10/12/17 from commons.wikimedia.org